We present a 60-year record of the stable isotopes of atmospheric carbon monoxide (CO) from firn air samples collected under the framework of the North Greenland Eemian Ice Drilling (NEEM) project. CO concentration, δ¹³C, and δ¹⁸O of CO were measured by gas chromatography/isotope ratio mass spectrometry (gc-IRMS) from trapped gases in the firn. We applied LGGE-GIPSA firn air models (Witrant et al., 2011) to correlate gas age with firn air depth and then reconstructed the trend of atmospheric CO and its stable isotopic composition at high northern latitudes since 1950. The most probable firn air model scenarios show that δ¹³C decreased slightly from −25.8‰ in 1950 to −26.4‰ in 2000, then decreased more significantly to −27.2‰ in 2008. δ¹⁸O decreased more regularly from 9.8‰ in 1950 to 7.1‰ in 2008. Those same scenarios show CO concentration increased gradually from 1950 and peaked in the late 1970s, followed by a gradual decrease to present day values (Petrenko et al., 2012). Results from an isotope mass balance model indicate that a slight increase, followed by a large reduction, in CO derived from fossil fuel combustion has occurred since 1950. The reduction of CO emission from fossil fuel combustion after the mid-1970s is the most plausible mechanism for the drop of CO concentration during this time. Fossil fuel CO emissions decreased as a result of the implementation of catalytic converters and the relative growth of diesel engines, in spite of the global vehicle fleet size having grown several fold over the same time period

We present a 60-yr record of atmospheric CO concentration and stable isotopic ratios at high northern latitude based on firn air samples collected in the frame of the North Greenland Eemian Ice Drilling (NEEM) project. Concentration, δ¹³C, and δ¹⁸O of CO from trapped gases in the firn were measured by gas chromatography coupled with isotope ratio mass spectrometry (gc-IRMS). Using models of trace gas transport in firn, the long-term trend of atmospheric CO and its stable isotopic composition at high northern latitudes since the 1950s were reconstructed. Our best firn air scenarios suggest that δ¹³C decreased slightly from −25.8‰ in 1950 to −26.4‰ in 2000, then dropped to −27.2‰ in 2008. δ¹⁸O decreased more regularly from 9.8‰ in 1950 to 7.1‰ in 2008. The best firn air scenarios also suggest that CO concentration increased gradually from 1950 and peaked likely in the late-1970s, followed by a gradual decrease by present day (Petrenko et al., 2011). An isotope mass balance model is applied to quantify the temporal evolution of CO source partitioning able to explain the combined mixing ratio and isotopic ratio changes. It suggests that a slight increase followed by a large reduction in CO derived from fossil fuel combustion occurred since 1950. The increase of CO concentration from 1950 to the mid-1970s is the result of a combined increase of multiple sources. The reduction of CO emission from fossil fuel combustion after the mid-1970s is the most plausible mechanism for the drop of CO concentration during this time. The mitigation policy for CO emission from vehicle exhaust such as application of catalytic converters and the growth of diesel engine vehicles market share are the main expected reasons for the CO source strength change from fossil fuel combustion

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